I had to re-install Kate.
I had this option, when writing javascript, and situation my cursor behind a bracket, ita nd the the corresponding one(opening or closing) become bold.
the effect looked like this:

function {
some code here
}
Does anyone know which setting is responsible? I had to re-install and seem I cannot find it.

It's probably working, just that the default bracket highlight is very faint. Try to change the color in Settings -> Configure Kate... -> Editor Component -> Fonts & Colors -> Bracket highlight.

Not sure why they don't become bold anymore and why the default color is so close to the background color. Personally I consider it a bug.

I use Kate inside Ubuntu, not Kubuntu. That might be the problem. I cant find the setting you are talking about - All I find there is a Color tab, and the setting you are talking about gives me the option to select color.
The selected color colors the whole selected function - I might be using newer version or very old version of Kate... This does the trick at the moment, so I will live with it

This is probably because you've enabled "Highlight range between selected brackets" in Settings -> Configure Kate... -> Editor Component -> Appearance (Advanced).

You are right. Deselecting the options really makes the brackets look a little bit bolder. It seems the ubuntu font is to blame, or some ubuntu setting.
Thank you for the help

Thanks for sharing. It is really nice info.